Ultrasonic plasticity of metallic glass near room temperature

Bulk metallic glasses (BMGs) are well-known for their superb strength (1–4 GPa) (Ashby and Greer, 2006) [1] but poor/localized plasticity when deformed at low temperatures or high strain rates (Inoue and Takeuchi, 2011; Kumar et al., 2009) [2,3]. Therefore, processing of BMGs, such as forming and shaping for various important applications, is usually performed above their glass transition temperatures (T_g) – also known as “thermo-plastic” forming (Geer, 1995) – for which the selection of alloy composition and the protocol for thermal treatment is demanding in order to promote extensive homogeneous plastic flows while avoiding crystallization (Geer, 1995). In stark contrast, here we demonstrate that homogeneous super-plasticity can occur rapidly in different BMGs below their T_g when subjected to ultrasonic agitations. Through atomistic simulations and nanomechanical characterization, we provide the compelling evidence to show that this super-plasticity is attributed to dynamic heterogeneity and cyclic induced atomic-scale dilations in BMGs, which leads to significant rejuvenation and final collapse of the solid-like amorphous structure, thereby leading to an overall fluid-like behavior. Our finding uncovers that BMGs can undergo substantial plastic flows through unusual liquefaction near room temperature and, more importantly, it leads to the development of a facile and cost-effective “ultrasonic-plastic” forming method to process a wide range of BMGs at low temperatures.